This paper presents the behaviour and design of axially loaded elliptical steel hollow sections filled with normal and high strength concrete. The experimental investigation was conducted with three nominal wall thickness (4mm, 5mm and 6.3mm) and different infill concrete cube

strengths varied from 30 to 100 MPa. The effect of steel tube thickness, concrete strength, and confinement were discussed together with column strengths and load-axial shortening curves were evaluated. The study is limited to cross-section capacity and has not been validated at

member level. Comparisons of the tests results together with other available results from the literature have been made with current design method used for the design of composite circular steel sections in Eurocode 4 and AISC codes. It was found that existing design guidance for

concrete filled circular hollow sections may generally be safely applied to concrete filled elliptical

steel tubes

Lam, D.

Testo, N.

English

White Rose Research Online

promoting access to White Rose research papers    White Rose Research Online   Universities of Leeds, Sheffield and York http://eprints.whiterose.ac.uk/     White Rose Research Online URL for this paper: http://eprints.whiterose.ac.uk/8482/    Conference paper Lam, D. and Testo, N. (2008) Structural design of concrete filled steel elliptical hollow sections. In: Composite Construction VI, July 2008, July 20-24, 2008, Devil's Thumb Ranch, Colorado, USA.  eprints@whiterose.ac.uk  Composite Construction VI, July 2008, Colorado, USA.     STRUCTURAL DESIGN OF CONCRETE FILLED STEEL ELLIPTICAL HOLLOW SECTIONS  Dr Dennis Lam School of Civil Engineering University of Leeds, LS2 9JT, UK e-mail:  d.lam@leeds.ac.uk   Miss Nicola Testo School of Civil Engineering University of Leeds, LS2 9JT, UK e-mail:  n.testo@leeds.ac.uk    ABSTRACT  This paper presents the behaviour and design of axially loaded elliptical steel hollow sections filled with normal and high strength concrete. The experimental investigation was conducted with three nominal wall thickness (4mm, 5mm and 6.3mm) and different infill concrete cube strengths varied from 30 to 100 MPa. The effect of steel tube thickness, concrete strength, and confinement were discussed together with column strengths and load-axial shortening curves were evaluated. The study is limited to cross-section capacity and has not been validated at member level. Comparisons of the tests results together with other available results from the literature have been made with current design method used for the design of composite circular steel sections in Eurocode 4 and AISC codes. It was found that existing design guidance for concrete filled circular hollow sections may generally be safely applied to concrete filled elliptical steel tubes.   INTRODUCTION Concrete filled steel tube (CFST) columns are becoming increasingly popular due to the advantages they offered. They are not only considered aesthetically pleasing but can also offer significant improvement in axial capacity without increases in cross-sectional area being required. Elliptical steel hollow sections represent a recent and rare addition to the range of cross-sections available to structural engineers, however, despite widespread interest in their application, a lack of verified design guidance is inhibiting uptake. The use of elliptical steel hollow section (EHS) with concrete infill is new and innovative, not only provides the advantage mentioned above, but also on the basis of both architectural appeal and structural efficiency. The role of the concrete core is not only to resist compressive forces but also to reduce the potential failure of local buckling by the steel sections. Although local buckling of the steel sections is reduced by the presence of the concrete core, local buckling will still occur to some extent. The concrete core will prevent the steel sections buckling inwards and therefore buckling Composite Construction VI, July 2008, Colorado, USA.  will generally occur by the outwards buckling of the steel tube. In turn, the steel section reinforced the concrete to resist any tensile forces, bending moments and shear forces and provides confinement to the concrete core. Composite members are of significant interest not only for this reason alone but also for their design suitability in seismic zones [Hajjar, 2000] and their inherent fire resistance [Han et al., 2003]. Existing research on EHS are mainly focused on its behaviour in compression and bending [Gardner and Chan 2007, Chan and Gardner 2007, Zhu and Wilkinson 2006] and also in welded connections [Choo et al 2003, Pietrapertosa and Jaspart 2003, Willibald et al 2006]. Previous research into the structural behaviour and design of concrete filled elliptical hollow sections has been rather limited, with the only reported studies to date by [Zhao et al., 2007]. The results from this study are analysed in this paper.   EXPERIMENTAL STUDY In order to investigate the behaviour of the elliptical CFSTs, 12 specimens were tested. 150 × 75 mm hot-finished elliptical hollow section (EHS) with 4.0mm, 5.0mm and 6.3mm wall thickness and three nominal concrete strength – C30, C60 and C100 were used for the tests. All specimens were 300mm in length to reduce end effects and to ensure that the specimens behaved as stub columns with little effect from column slenderness. The properties for all the specimens are listed in Table 1.   Table 1: Measured geometric properties of the specimens Reference Major axis,  outer diameter, 2a  (mm) Minor axis,  outer diameter, 2b  (mm) Wall thickness,  t  (mm) Steel area, As (mm2) Concrete area, Ac (mm2) 150×75×4 150.40 75.60  4.18 1471.5 - 150×75×4-C30 150.40 75.60 4.18 1471.5 7458.6 150×75×4-C60 150.57 75.52 4.19 1475.8 7455.0 150×75×4-C100 150.39 75.67 4.18 1471.8 7466.0 150×75×5 150.23 75.74 5.08 1773.8 - 150×75×5-C30 150.12 75.65 5.12 1785.5 7133.9 150×75×5-C60 150.23 75.74 5.08 1773.8 7162.8 150×75×5-C100 150.28 75.67 5.09 1777.1 7154.2 150×75×6.3 148.78 75.45 6.32 2164.1 - 150×75×6.3-C30 148.78 75.45 6.32 2164.1 6652.3 150×75×6.3-C60 148.92 75.56 6.43 2202.1 6635.5 150×75×6.3-C100 149.53 75.35 6.25 2149.1 6700.0  Testing of the composite columns was carried out using a 3000kN capacity ToniPACT testing machine and the experimental set up is shown in Figure 1. Both ends of the specimens were milled flat and capped with rigid steel plate in order to distribute the applied load uniformly over both the concrete and steel section for the composite loaded columns. The specimens were loaded at 50kN intervals at the beginning of the test (i.e. in the elastic region) and at a loading Composite Construction VI, July 2008, Colorado, USA.  rate of 10kN intervals after the column began to yield, in order to have sufficient data points to delineate the “knee” of the stress-strain curve. A linear variable differential transducer (LVDT) was used to monitor the vertical deformation. All of the operation and the change of loading rate were operated manually and all the readings were recorded when both load and strain had been stabilized. After the immediate drop of the load due to local buckling, the test continued until excessive deformation of the column was observed. After the test, the specimens were removed, photographed and carefully examined.   Figure 1: Test arrangement and instrumentations  Concrete Properties Three nominal concrete strengths – C30, C60 and C100 were studied. The concrete was produced using commercially available materials with normal mixing and curing techniques; the three mix designs are shown in Table 2 together with the cube and cylinder strength at test day. The strength development of the concrete was monitored over a duration of 28 days by conducting periodic cube and cylinder tests – the results of the cube tests are illustrated in Figure 2. Additionally, at the time of each series of stub column tests, two further standard cube tests and two standard cylinder tests were performed.   Composite Construction VI, July 2008, Colorado, USA.  Table 2: Concrete mix proportions (% by weight) and the compressive strength (test day) Grade Cement Fines Coarse w/c ratio Silica fume Super-plasticiser fcu (N/mm2) fck (N/mm2) C30 1.0 2.5 3.5 0.65 0 0 36.9 30.5 C60 1.0 2.0 3.3 0.40 0 0 59.8 55.3 C100 1.0 1.5 2.5 0.30 0.1 0.03 98.4 102.2  01020304050607080901000 5 10 15 20 25 30Time (Days)Concrete Cube Strength (MPa)C100C60C30Figure 2: Concrete development strength   Steel Properties Coupons were cut from the EHS and tested to [EN10002-1 2001] to determine the tensile strength. The coupons were cut form the in the region of maximum radius of curvature (i.e. the flattest portion of the section) and milled to specification. Some flattening of the ends occurred while gripping the specimen but this was well away from the ‘neck’ of the sample. The results from the coupon tests are summarized in Table 3. Table 3: Steel properties of the EHS Specimens Young’s modulusE (N/mm2) Yield stress fy (N/mm2) Ultimate strength fu (N/mm2) 150×75×4 217500 376.5 513 150×75×5 217100 369.0 505 150×75×6.3 216500 400.5 512 Composite Construction VI, July 2008, Colorado, USA.  TEST RESULTS All the specimens were tested under axial compression until failure. The typical failure modes of the composite specimens are shown in Figure 3. For the unfilled EHS, both inward and outward local buckling was observed in the deformed specimen while for the filled tubes, although inward buckling was prevented by the concrete core, outward local buckling is clearly evident in the deformed specimens.    Figure 3: Typical failure mode of the composite EHS  The load vs. end shortening curves from the EHS stub column tests are shown in Figures 4 to 6. The results show the clear advantage of composite EHS columns over their bare (unfilled) EHS counterparts. Overall, it may be observed from Figures 4 to 6 that the stockier EHS tubes with lower concrete strengths have more ductility, though enhancements in load carrying capacity beyond that of the bare steel sections due to concrete filling are more significant for slender sections with higher concrete strengths. The ultimate loads from the stub columns tests Nu,Test are presented in Tables 4 with the composite factor, φ. The level of strength enhancement (beyond that of the unfilled tubes) can be represented by the composite factor, φ, the definition of which is given by Eq. (1). This index provides a quantitative measure of the benefit arising from concrete-filling.  Composite Construction VI, July 2008, Colorado, USA.  020040060080010001200140016000 5 10 15 20 25 30End Shortening (mm)Axial Load (kN)C100C60C30EmptyFigure 4: Axial load vs. end shortening curves for 150×75×4 EHS composite columns   020040060080010001200140016000 5 10 15 20 25 30End Shortening (mm)Axial Load (kN)C100C60C30EmptyFigure 5: Axial load vs. end shortening curves for 150×75×5 EHS composite columns   Composite Construction VI, July 2008, Colorado, USA.  020040060080010001200140016000 5 10 15 20 25 30End Shortening (mm)Axial Load (kN)C100C60C30EmptyFigure 6: Axial load vs. end shortening curves for 150×75×6.3 EHS composite columns  Table 4: Summary of test results and composite factor, φ Reference Nu, Test Composite factor, φ 150×75×4 550.0 1.00 150×75×4-C30 838.6 1.52 150×75×4-C60 974.2 1.77 150×75×4-C100 1264.6 2.30 150×75×5 688.9 1.00 150×75×5-C30 981.4 1.42 150×75×5-C60 1084.1 1.57 150×75×5-C100 1296.0 1.88 150×75×6.3 871.8 1.00 150×75×6.3-C30 1202.9 1.38 150×75×6.3-C60 1280.1 1.47 150×75×6.3-C100 1483.2 1.70  unilledu,filledu,NN=φ        (1) Where, Nu,filled is the ultimate resistance of the concrete-filled elliptical test specimens;  Nu, unfilled is the ultimate test resistance of the corresponding empty EHS. Composite Construction VI, July 2008, Colorado, USA.  Figure 7 shows the relationship between the composite factor, φ and the cube strength of the concrete fcu for the three different tube thicknesses. The results show that, as expected, the concrete contribution ratio increases for the higher concrete strengths, and that the level of enhancement is more significant for the thinner tubes; the 4mm elliptical tube shows a doubling in capacity with the C100 concrete infill.   11.21.41.61.822.22.420 40 60 80 100 120Concrete Cube Strength, fcu (MPa)Composite Factor, φt = 4.0mmt = 5.0mmt = 6.3mm Figure 7: Composite factor vs. concrete cube strength curves   DESIGN CODE Concrete-filled elliptical hollow sections are not explicitly covered by current design codes. The test results obtained in the present study have been combined with those reported by [Zhao et al., 2007] and compared with existing design guidance for the circular concrete-filled tubes. The codes considered are [EN 1994-1-1 2004] and [AISC 360-05 2005] respectively abbreviated to EC4, and AISC in this paper. The principal differences between the codes relate to the factors that are applied to the individual steel and concrete contributions to the composite resistance. Following the comparisons, design recommendations are made for concrete-filled elliptical hollow sections.   EC4 covers concrete encased and partially encased steel sections and concrete-filled tubes with and without reinforcement. The compressive resistance Nu,EC4 of concrete-filled steel tubes is given by Eq. (2). This is the latest design code that takes into account increases in concrete capacity due to confinement by the steel sections.   Composite Construction VI, July 2008, Colorado, USA.       (2)  Where, Nu,EC4  Ultimate axial capacity of the composite column  fcd  Design compressive strength of the concrete fck  Cylinder strength of concrete  fy  Yield strength of the steel tube fyd  Design strength of the steel tube d  Larger diameter of the elliptical steel section t  Thickness of steel tube ηc  Coefficient of concrete confinement ηa  Coefficient of steel confinement  In the AISC code, the compressive resistance of concrete-filled circular hollow sections Nu,AISC is given by Eq. (3). The 0.95 factor on the concrete contribution in Eq. (3) reflects the superior performance of concrete-filled CHS over their rectangular counterparts.  ckcysAISC, 95.0 fAfANu +=      (3)   0.00.20.40.60.81.01.21.4N Code / N TestEC4 AISC Figure 8: Comparison of code prediction   ⎟⎟⎠⎞⎜⎜⎝⎛++=ckyccdcydaaECu ffdtfAfAN ηη 14,Composite Construction VI, July 2008, Colorado, USA.   Table 5: Comparison between test results and codes prediction Reference Nu, Test (kN) Nu, EC4 (kN) TestuECuNN,4,  Nu,AISC (kN) TestuAISCuNN,,  150×75×4-C30 838.6 871.7 1.04 770.1 0.92 150×75×4-C60 974.2 1046.6 1.07 947.3 0.97 150×75×4-C100 1264.6 1379.1 1.09 1279.0 1.01 150×75×5-C30 981.4 977.8 1.00 865.6 0.88 150×75×5-C60 1084.1 1140.7 1.05 1030.8 0.95 150×75×5-C100 1296.0 1459.2 1.13 1350.4 1.04 150×75×6.3-C30 1202.9 1184.6 0.98 1059.5 0.88 150×75×6.3-C60 1280.1 1354.2 1.06 1230.5 0.96 150×75×6.3-C100 1483.2 1630.1 1.10 1511.2 1.02 150×75×4-C60* 1075 1193.1 1.11 1087.9 1.01 150×75×5-C60* 1163 1229.5 1.06 1118.4 0.96 150×75×6.3-C60* 1310 1370.3 1.05 1247.8 0.95 200×100×5-C60* 1598 1991.3 1.25 1819.4 1.14 200×100×6.3-C60* 2068 2181.4 1.05 1989.5 0.96 200×100×8-C60* 2133 2404.7 1.13 2193.3 1.03 200×100×10-C60* 2290 2514.9 1.10 2331.2 1.02 *Test reported by [Zhao et al., 2007]  Mean 1.08 Mean 0.98   SD 0.061 SD 0.064  All the test results presented in this paper have been combined with those reported by [Zhao et al. 2007] and compared with the predictions from the aforementioned design codes. The comparisons, shown in Figure 8 and Table 5 reveal that the ultimate test loads from the 16 concrete-filled EHS specimens are generally over-predicted by the EC4 formulations for concrete-filled CHS by 8% and underestimated by 2% by the corresponding AISC concrete-filled CHS formulations. On the basis of the comparisons, it is recommended that the AISC expression for concrete-filled CHS (Eq. (3)) is most suitable for predicting the resistance of concrete-filled EHS. However, it is clear that the level of confinement and hence the resistance of concrete-filled EHS are related to the aspect ratio of the section and further research to investigate this feature is ongoing.   CONCLUSIONS A total of 12 tests – 9 compositely loaded and 3 unfilled EHS have been performed to investigate the compressive behaviour of concrete-filled elliptical hollow sections. The compressive response was found to be sensitive to both steel tube thickness and concrete strength, with higher tube thickness resulting in higher load-carrying capacity and enhanced Composite Construction VI, July 2008, Colorado, USA.  ductility, and higher concrete strengths improving load-carrying capacity but reducing ductility. The experimental results from the present study were combined with an additional 7 experimental results from literature, and compared with existing code provisions for circular hollow sections. From the comparisons, it may be concluded that existing design rules for concrete-filled CHS may be safely applied to EHS, and that the AISC design expression for CHS provide an accurate prediction of composite EHS behaviour.   ACKNOWLEDGEMENTS The authors would like to thank Dr Leroy Gardner of Imperial College for his time and help with the research work and Corus for providing the steel specimens used in this research project. The skilled assistance provided by the technical staff in the School of Civil Engineering at Leeds University is also appreciated.   REFERENCES AISC 360-05 (2005), Specification for structural steel buildings, American Institute of Steel Construction. Chan, T. M. and Gardner, L (2008), Compressive resistance of hot-rolled elliptical hollow sections, Engineering Structures, 30, pp. 522 – 532. Choo, Y.S., Liang, J.X. and Lim, L.V. (2003), Static strength of elliptical hollow section X-joint under brace compression, 10th. Int. Symposium on Tubular Structures, Madrid, Spain, pp. 253 – 258. EN 10002-1 (2001). Metallic materials – Tensile testing – Part 1: Method of test at ambient temperature, British Standard Institute. EN 1994-1-1 (2004), Eurocode 4: Design of composite steel and concrete structures - Part-1-1: General rules and rules for buildings, British Standard Institute. Gardner, L. and Chan, T. M. (2007), Cross-section classification of elliptical hollow sections, Journal of Steel and Composite Structures, 7(3), pp. 185 – 200. Hajjar, J.F. (2000), Concrete filled steel tube columns under earthquake loads, Progress in Structural Engineering and Materials, 3(1), pp. 72 – 81. Han, L.H., Yang, Y.E. and Xu, L. (2003), An experimental study and calculation on the fire resistance of concrete-filled SHS and RHS columns, Journal of Constructional Steel Research, 59(4), pp. 427 – 452. Pietrapertosa, C. and Jaspart, J.-P. (2003), Study of the behaviour of welded joints composed of elliptical hollow sections, 10th. Int. Symposium on Tubular Structures, Madrid, Spain, pp. 601 – 608. Composite Construction VI, July 2008, Colorado, USA.  Willibald, S., Packer, J.A. and Martinez-Saucedo, G. (2006), Behaviour of gusset plate connections to ends of round and elliptical hollow structural section members, Canadian Journal of Civil Engineering, 33, pp.373 – 383. Zhao, X. L., Lu, H. and Galteri, S. (2007). Tests of Elliptical Sections Filled with SCC (Self-Compacting Concrete). Proceedings of the 5th International Conference on Advances in Steel Structures. Singapore, pp. 950 – 955. Zhu, Y. and Wilkinson, T. (2006), Finite element analysis of structural steel elliptical hollow sections in pure compression, 11th. Int. Symposium on Tubular Structures, Québec City, Canada, pp. 179 – 186. 

Structural design of concrete filled steel elliptical hollow sections

This paper presents the behaviour and design of axially loaded elliptical steel hollow sections filled with normal and high strength concrete. The experimental investigation was conducted with three nominal wall thickness (4mm, 5mm and 6.3mm) and different infill concrete cube strengths varied from 30 to 100 MPa. The effect of steel tube thickness, concrete strength, and confinement were discussed together with column strengths and load-axial shortening curves were evaluated. The study is limited to cross-section capacity and has not been validated at member level. Comparisons of the tests results together with other available results from the literature have been made with current design method used for the design of composite circular steel sections in Eurocode 4 and AISC codes. It was found that existing design guidance for concrete filled circular hollow sections may generally be safely applied to concrete filled elliptical steel tubes

Lam, Dennis

Bradford Scholars

Structural Design of Concrete Filled Steel Elliptical Hollow Sections

https://eprints.whiterose.ac.uk/8482/1/Lamd1.pdf

Structural design of concrete filled steel elliptical hollow sections

Abstract

Similar works

Full text

Available Versions

White Rose Research Online

Bradford Scholars

White Rose Research Online